The present invention relates to optical measurement systems and more particularly to a method and apparatus for correcting errors in an output signal of a detector array caused by distortions which exist in the lens or lens system which conducts radiation to the detector array.
In most lenses, particularly those comprised of a series of individual lenses so arranged as to provide a desired characterized output, certain unavoidable distortion errors are found. "Distortion" as used herein means the variation in magnification with different distances off axis. That is, as the object varies in position from the optical axis, the image position varies in a non-linear manner. Manufacturers of lenses are able make lenses which have an exact desired magnification along the optic axis, for example, but as the magnification is checked at points more remote from the optic axis, slight differences from the desired magnification are almost always found. These distortions are for the most part symmetrical about the optic axis and are found to follow a curve which is substantially the same for all lenses of a particular design, but the distortions from one design to another vary. The equation for the curve representing the distortion is derivable from an examination of the lenses and the differences from one type of lens to another can also be found. In the prior art, however, the user of the lens in, for example a measurement system, has not known the distortion values and has not been able to correct the errors particularly when the lenses he uses must be interchangeable from one type to another type requiring different corrections. I have also discovered that the distortion of the lens effects the measurement system in slightly different but significant ways for varying distances to the object being detected. Since the distortions vary with distance, distortion compensation based on a reference distance will be incorrect for other distances which may include the actual target distances. These distance dependent variations limit the achievable system measurement accuracy. Also I have found that there are certain inherent errors usually found in the measurement system or the camera which observes the object. Such errors, as for example, the non-centering of the detector array in the image of the object can introduce further distortions in the final output.